The overall objective of the proposed research is to elucidate in molecular terms the mechanism by which interferon acts to affect a wide variety of fundamental viral and cellular processes. The specific aims and methods are: (1) To partially purify by standard biochemical techniques the apparent interferon-mediated ribosome-associated polypeptide present in interferon-treated cells and normally absent in untreated cells, and then to determine the biochemical and biophysical properties of the purified polypeptide. (2) To elucidate the precise biochemical step in the translation of viral messenger RNA in vitro that is inhibited in cell-free systems prepared from interferon-treated cells but not in systems prepared from untreated cells; this will be performed by analysis of the rate and extent of reovirus-specific initiation dipeptide synthesis and aminoacyl-tRNA and messenger RNA ribosome binding, as well as kinetic examination of messenger RNA translation using various amino acids and specific antibiotic inhibitors (pactamycin, sparsamycin, trichodermin). (3) To resolve whether the inhibitory activity of the host-coded interferon-mediated factor is directed specifically toward viral messenger RNAs, or whether the translation of certain classes of cellular messenger RNAs are also selectively affected; this will be performed by studying the kinetics of translation in vitro of several different purified viral and cellular messenger RNAs that code for well characterized polypeptide products. (4) To determine the species-specificity of the host-coded interferon- mediated inhibitor of messenger RNA translation in vitro; this will be carried out by examining what effects ribosome salt-wash fractions and cell-sap fractions from untreated and homologous and heterologous interferon-treated mouse, human, and chicken cells have on the translation of viral and cellular mRNAs in vitro by a mouse cell-free system from untreated ascites cells.